I appreciate you coming to the Community for help, @tangoitequine.
We can isolate this first by re-sending the invoices to your clients and let them pay using the new ones. Once sent, have them check their email and make sure to choose Pay Now right from that invoice.
However, if the issue persists, I'd recommend letting your client use a different browser when paying their invoice. There are times wherein the stored data can cause some odd behaviors. If it works fine using a different portal, they can go back to their regular browser and clear the cache.
If they are getting the same result, you'll have to reach out to our QuickBooks Support so they can further isolate the issue.
Please leave a comment if you have any questions. I'm always here to provide further assistance. Have a great day!
